All current protein purification systems show deficiencies, especially for large-scale purifications. In particular, the purification of antibodies is inadequate as far as purity and yields are concerned. Consequently, the present state of the art processes are costly due to large volumes of solvents and long production times for making pure immunoglobulins intended for therapeutic and diagnostic applications.
In an affinity separation, the protein being purified adsorbs selectively and reversibly to a complimentary binding substance or affinity ligand, often times an antibody molecule. Affinity separation generally results in very low non-specific binding compared to other separation techniques. The very low non-specific binding makes it possible to purify a given protein from complex biological mixtures, to separate incorrectly folded forms from native molecules, and to recover the protein.
Purification systems using glass or metal tubes that contain a packed column of separation medium, for example, beads or particles, are known. These tubes are known as column boxes. Because the separation medium is compacted within the column boxes, the flow rates are slow and the column boxes have a limited capacity. Therefore, prior art purification technology has focused on increasing the porosity of the separation medium to increase the flow rates and capacity within the column box. The object of these known systems is to purify the largest amount of material within the shortest amount of time while keeping the amount of contaminants in the product low and the product yields high. One problem with the old purification technology is that increasing porosity of the separation medium achieved faster flow rates and capacity, but reduced product yields and purity.
Orbicell(trademark) cellulose beads are available from Accurate Polymers, Ltd. in Illinois and described in U.S. patent application Ser. No. 09/324,527 filed on Jun. 2, 1999 to Stipanovic et al. entitled Static Separation Method Using Non-Porous Cellulose Beads, incorporated herein by reference.
A typical affinity adsorbent consists of a solid support, a spacer arm, and a ligand. The spacer arm may encourage protein binding by making the ligand more accessible. Certain affinity separation products do not have a spacer arm of sufficient length or suitable nature so as to aid in the attachment of the target compound, for example, an antibody. In order to overcome this limitation, there is a need for a spacer arm that would allow for better orientation of the ligand, decreased steric hindrance between the target compounds, and decreased steric hindrance between the ligands thereby allowing for greater attachment of the target compound. Ease in attachment of the target compound to the ligand due to the geometry of the spacer arm and/or ligand increases target compound yield. There is a need in the art for the ligand to exhibit specific and reversible binding to the target compound, for example, a protein such as an antibody.
Affinity chromatography on immobilized Staphylococcus Aureus Protein A (SpA), Protein G and Protein M columns is a recent purification method for monoclonal and polyclonal antibody production. These bacteria-derived proteins are not only costly to produce, but also suffer from biological and chemical instability. An ability to be cleaned and sterilized is an absolute requirement by regulatory authorities for sorbents used to purify antibodies destined for therapeutic end use. Polyclonal antibodies, as well as more recently developed monoclonals, are routinely purified by affinity column chromatography. These antibodies have wide applications in diagnostic field, but lately similar antibodies are more and more finding their use in therapeutic applications. The latter application can hardly tolerate any instability of affinity sorbents, which becomes especially critical when steam, or harsh chemical sterilization procedures are mandated by regulatory agencies.
Of course, the therapeutic end-use will create a demand for much larger quantities of highly purified antibodies than the diagnostic field ever did. The first obstacle that the present state of the art-technology faces on any future scale-ups are the high cost of these bacteria-derived proteins. On top of the high cost of sorbents, will be the regulatory authorities thorough scrutiny of potential instabilities of sorbents during the required sterilization protocols. Therefore, there is a need in the art to solve these and other difficulties of the prior art.
Recent advances in molecular modeling have enabled research groups to come up with much smaller molecules than bacteria-derived proteins, which surprisingly, can still simulate high affinity and selectivity of respective bacterial proteins for numerous immunoglobulins. Said model synthetic compounds possessing affinity for antibodies, range from simple monocyclic and polycyclic compounds, to peptides of short to medium length. Some of these peptides are linear, others have macrocyclic structure.
The present invention is directed to novel ligands, a method of preparing the ligands, a method of using the ligands in the purification of compounds, and the attachment of the novel ligands to cellulose beads.
The large-scale purification of bio-molecules and, in particular, immunoglobulines, is accomplished by using a cellulose bead attached to small, non-peptidic compounds which display a high affinity and selectivity for the bio-molecule to be purified. In addition, the beads with the attached ligands of Formulas I-IV also possess a high chemical stability under rigors of recycling and sterilization. A method of purifying a compound includes providing a support matrix having a ligand of Formula I-IV thereon to a solution containing a compound to be separated, allowing for interaction of the ligand and compound to be separated, and washing the support matrix to elute the compound to be separated. In one embodiment, the support matrix may include a spacer.
The invention will best be understood by reference to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings. The discussion below is descriptive, illustrative and exemplary and is not to be taken as limiting the scope defined by any appended claims.
In one embodiment, Formula I shows a non-peptidic, small compound that simulates the affinity of Protein A for immunoglobulins. A compound which simulates the affinity of Protein A can also be referred to as a protein A mimetic (PAM). As shown in Reaction 1, ligands of Formula I can be prepared and immobilized on Orbicell(trademark) beads having a spacer surface chemistry terminated with an amino group on a linear polyethylene glycol molecule of molecular weight with ranging between 64 and 2000 daltons. Hydrazine can be reacted with the terminal esters resulting in a molecular super-structure which is terminated with hydrazide functionalities (a). In a further embodiment, the molecular super-structure has a terminal functional group selected from the group consisting of succinimidyl, thiophenyl, 2,4-dimethoxy-s-triaznyl, cynomethyl, chloroformyl, and para-nitrophynel esters. The cellulose beads can range in size from about 0.1 microns to 20 microns. The molecular super-structure improves the geometry of the resulting attached ligands. The two terminal hydrazine groups of the molecular super-structure allow for a plurality of ligands to attach.
In one preferred embodiment, the immunoglobin to be separated is IgG. A hydrophobic interaction occurs between the IgG molecule and a mimetic SpA ligand attached to a support matrix, such as Orbicell(trademark) beads. In this manner, the IgG is separated from the solution to which are added the non-porous, cellulose beads with the attached mimetic SpA ligands. The IgG attaches to the mimetic SpA on the beads. The beads with the resulting ligand-IgG complex are washed of impurities so that a subsequent elution results in IgG and reusable beads having a regenerated ligand for repeated binding with IgG. The present ligand more easily attaches to the support matrix and has a greater reactivity with the IgG.
As shown in Reaction 1, once terminated with hydrazide functionalities, an s-triazinyl moiety (b) can be attached. The reaction with the hydrazine was achieved at the dichloro-s-triazine stage, as the second chloride is of much higher reactivity than the third chloride. Since all of the chlorides are not reacted, the compound is highly reactive and high reactivity increases yield. Once attached on the bead surface, the resulting monochloro triazine derivative (c) was forced to react with an excess of an amino compound (d) at a higher reaction temperature than used in other stages of the reaction.
The present ligand of Formula I is attached to the solid state complex (the support matrix or, for example, the Orbicell(trademark) bead) under better spatial control and has a greater reactivity with the IgG (or other target compound to be purified). At least these factors increase the total yield of the purified IgG. The hydrazine moiety on the cellulose surface reacts readily with the very reactive chloride of dichlorotriazine molecule. The desired molecular geometry of two adjacent ligand groups should result in multiple points of attachment of the IgG to the ligands. The hydrazine represents the shortest bifunctional NH2 group linker which allows optimal spatial arrangement and proximity of the ligands. Therefore, the addition of the improved ligand with the improved spatial arrangement on the surface, in conjunction with improved non-porous Orbicell(trademark) bead, increases the interaction between the matrix and the IgG (i.e., other antibody or target compound). 
Very high levels of substitution of the monochloro triazine derivative were achieved. The ligand of Formula I was tested for affinity separation of antibodies. The sorbent displayed very high activity (yield of antibody was 44 mg/g of beads) and selectivity (pure antibodies by HPLC: no xe2x80x9cshouldersxe2x80x9d of impurities present).
In another embodiment, an alternative ligand that simulates Protein A activity toward immunoglobulin is derived from a meta phthalic acid derivative. As shown in Reaction 2, a molar excess of dimethyl-5-nitroisophthalate is reacted with tiramine in methanol and monoamide is isolated and crystallized (a). 5-Nitro monoamide of isophthalic acid ester is further reacted with excess of aniline, providing mixed 5-nitro bisamide (b). Hydrogenation over Pd catalyst under H2 pressure gives mixed 5-amino bisamide compound (c), SA-B. 
Compound SA-B can now be grafted onto the surface of cellulose beads having a spacer arm that is carboxyl functionalized activated via N-succinimidyl ester, as shown, resulting in ligand of general Formula II.
In another embodiment, a method of manufacturing a ligand of Formula II includes reacting a (4xe2x80x2 hydroxy)phenetyalmido-1-carboxy-anilido-3-carboxyphenyl-5-amine with a terminal, activated ester functional group. 
In another embodiment, a method of manufacturing a ligand of Formula III includes reacting a (a(4xe2x80x2 hydroxy) phenetyalmido-1-carboxy-anilido-3-carboxyphenyl-5-amine with a terminal tertiary dicarboxy-ethyl amine in the presence of a peptide-coupling agent.
Another alternative grafting method is based on coupling of carboxyl-functionalized beads with amino SA-B using a conventional carbodimide reagent, as shown in Reaction 4, resulting in the ligand of general Formula III shown below. 
A further embodiment of the adsorbents include a non-porous cellulose bead having a diameter of approximately 0.1 to 20xcexc being pegylated on the surface with xcex1,xcfx89-diamino polyethylene glycol groups ranging in molecular weights up to approximately 60 to 2000 Daltons. The primary amines are reacted with a triepoxide to form an amino-2-hydroxy adduct. The remaining epoxides are reacted with the thiol (mercapto) heterocyclic compounds wherein the R group includes pi electron rich systems resulting in the ligands of general Formula IV. The mercapto heterocyclic compounds may be selected from the group comprising mercapto-N-methyl imidiazol, 2-mercapto pyridine, mercaptopyridine-N oxide, 2-mercaptoimidazole, 2-mercaptobenzimidazole, sodium 2-mercapto-5-benz-imidazolesulfonic acid, 2-mercapto-benzothiazole, 2-mercaptobenzoxazole, 2-mercapto-5-methylbenzimidazole, 2-mercapto-1-methylimidazole, 2-mercapto-4-methylpyrimidine, 2-mercapto-5-nitrobenzimidazole, 2-mercaptopyridine, 2-mercaptopyridine N-oxide, 2-mercapto-pyrimidine, 2-mercapto-4(3H)-quinalozine, 2-mercaptothiazoline, 2-mercapto-thiazole, 2-mercaptothiadiazole, and 5-methyl-1,3,4-thiadiazole-2-thiol and other mercapto heterocyclic compounds known to those skilled in the art. The grafted mercapto heterocyclic compounds selectively bind to biological molecules, including IgM, IgY (egg), Fab, and Fc antibody fragments in addition to the whole IgG molecule. 
One skilled in the art could accomplish substitution of the phenol groups of Formulas I-IV with a thiol, amide or other equivalents. Additionally, salts of Formulas I-IV are included.
In one embodiment, the method of purification includes providing a mimetic Protein A (SpA) capable of binding immunoglobin. The mimetic Protein A is attached to a support matrix. In a prefered embodiment, the support matrix is a cellulose bead. In a preferred embodiment, the immunoglobin to be separated is IgG. An intereaction occurs between the IgG molecule and the mimetic SpA ligand. The beads with the resulting ligand-IgG complex are washed of impurities so that a subsequent elution results in pure IgG and reusable beads having a regenerated ligand for repeated binding with IgG.